The invention relates to a method of concentrating potassium or sodium hydroxide solution liquid by means of a heated evaporator and an apparatus for carrying out the method.
The concentration of sodium or potassium hydroxide liquids is characterised in that on the one hand there are high boiling points and therefore it is necessary to use heating media with suitably high heating temperatures, and on the other hand the transfer of the applied heat to the liquid to be concentrated requires a considerable outlay on expensive material, more particularly pure nickel, for the heat transfer surfaces. In modern methods which are already known, concentration takes place in tube bundle evaporators, and outside the tubes there is contact with a condensing or liquid heating medium of sufficient temperature, whereas in the interior of the tubes the liquid to be concentrated flows through in a once-through system and in concurrent flow with the evaporation vapours, either upwards from below as in a rising flow tube evaporator or downwards from above as in a down flow tube evaporator.
Associated with the outlet from the tube bundle is a separating system in order to separate the liquid vapour mixture into the individual components and to discharge the two components individually.
The issuing evaporation vapours are considerably superheated in accordance with the high boiling temperature, and as a result represent the most important quantity of heat which is discharged during the concentration process.
A known method of keeping these heat losses as small as possible, consists in carrying out the concentration operation under a vacuum and as a result the outlet temperature of liquid and vapours are approximately 80.degree. C lower.
According to another known method the concentration operation is subdivided into one or more preliminary stages so that the result is achieved that only the vapour part-quantity of the last stage issues with the high superheating temperature.
In another known method, the superheated vapours issuing from the end stage at approximately atmospheric pressure are used for heating a pre-concentration stage, the vapours which form in the latter being precipitated in a condenser which is operated under a vacuum, and the cooled vapours of the end stage being delivered into the atmosphere.